1. Field of the Invention
The present invention relates generally to valve actuator systems and particularly to valve actuator systems using two stroking speeds for the actuation, fast and slow, depending on the degree of required position adjustment of the actuator.
2. Description of the Prior Art
Known valve actuators utilize a single stroking speed and are usually pneumatic, electrohydraulic or electromechanical. Each has its own deficiencies which the present invention overcomes. Pneumatic actuators are necessarily large in size to provide the required force or thrust and suffer from dynamic instability because of the compressible nature of the pneumatic fluid, as well as from overshoot and instability due to the single stroking speed attempting to effect a small actuator position movement. Electromechanical and electrohydraulic actuators although having less sources of instability are usually slow in operation in comparison to the pneumatic actuators.
Examples of such known prior art devices may be found in the following U.S. Pat. Nos. 3,603,083 and 2,938,347.